When an electric generator is driven by an internal combustion engine, the engine speed and hence the output voltage of the generator is cyclically higher during power strokes of the engine than during exhaust strokes. If electric lights are supplied with current from such generator, the cyclical voltage variation causes a noticeable variation in light intensity which is visible as flicker. Of particular concern is a variation in speed with a periodic frequency of 15 Hz. At such frequency, light flicker is much more noticeable than at 30 Hz. Hence, a generator driven by an engine having a power stroke at a 15 Hz rate will produce visible flicker in lights supplied with current from the generator.
Lights supplied with power by a generator driven by an internal combustion engine are frequently used on construction jobs and in other circumstances where current from commercial power lines is not available. Such lights may, for example be metal arc lights which provide high intensity elimination. Continual flicker in such lights is distracting and annoying to workmen and other persons in the illuminated area.